For certain applications synthetic textile filaments and fibers are mixed with lower-melting synthetic binder filaments or fibers which, when properly heated, soften or melt to provide interfilament or interfiber bonding which stabilizes the fibrous structure. The use of copolyester binder fibers in fiberfill batts is described in U.S. Pat. Nos. 4,129,675 (Scott) and 4,068,036 (Stanistreet) and also in Research Disclosure, September 1975, Article No. 13717, page 14. The use of copolyester binder filaments for consolidating nonwoven webs and sheets is described in U.S. Pat. No. 3,989,788. These copolyester binders obtain their binder properties through replacement of some terephthalate repeating units in poly(ethylene terephthalate) with isophthalate units.
To modify poly(ethylene terephthalate) by copolymerization for use in films or fibers having a desired modified thermal response, it has commonly been considered preferable to employ a diacid comonomer rather than a glycol comonomer. Such preference is represented, for example, by the use of isophthalate copolymer units in binder filaments and fibers referenced above. This preference is also taught in U.S. Pat. No. 3,554,976 (Hull) which discloses copolymers of poly(ethylene terephthalate) with diethylene glycol (DEG) for films but it further teaches that replacement of some of the terephthalate repeating units with another diacid gives a desirable change of glass transition temperature combined with a minimal melting point depression. Inclusion of some azelate units provides more desirable properties than poly(ethylene terephthalate) modified with the diethylene glycol alone. This failure to appreciate any utility for poly(ethylene terephthalate) containing a large amount of diethylene glycol units is further substantiated in U.S. Pat. No. 4,025,592 on texturing yarns where the diethylene glycol content is limited to less than 4 mol percent to avoid undesirable effects on yarn properties.
Objects of this invention include improved copolyester binder filaments and fibers which provide effective bonding over a broad range of temperatures which range extends above and below their melting points, which are made from inexpensive readily available monomers and which can be prepared by polymerization and melt spinning using conventional apparatus designed for poly(ethylene terephthalate).